@gladmo makes a great point about the lower moving mass of the paper cone version of the F12, and the resulting reproduction of low frequency fine detail.
I have a pair of the Rythmik Audio/GR Research OB/Dipole Subs, each of which employs a pair of the paper cone woofer that has been optimized for open baffle use. Imagine the "speed" (settling time) of the F12 servo-feedback woofer, with no enclosure! It's the only woofer/sub I've ever heard that blends well with dipole planar loudspeakers (including a pair of KEF B139 woofers in transmissionline enclosures I own). If you've heard the Magnepan bass panels of the big Tympani and MG30.7 models you have an idea of the sound of the OB/Dipole Sub, but with reproduction of even the bottom octave (20-40Hz). But an OB/Dipole sub is not for everyone, or every application. For those there is the Rythmik Audio F12. I have no doubt a lot of REL and JL Audio owners would be very surprised if they ever heard a Rythmik sub.